More information about the origin of the radiation in LMR and IMR can be extracted from the high-statistics NA60 data.
In the lower right panel, the respective data are compared to the same Rapp/Wambach model, yet with a more elaborate tuning of the fireball expansion model.
This has been achieved by addressing at the same time dimuon invariant mass and transverse momentum spectra.
Most importantly, the expansion velocity of the fireball has been increased as to better match the slope of the \(p_t\)  distributions.
For details about the expansion model see [1]}.
Compared to the calculation shown in Fig. REF , which is based on the same emissivity but uses an earlier parameter set for the expansion model and focusing on \(\rho \)  contributions only, here also the high-mass region of the experimental distribution is well reproduced.
The successful description of the dimuon yield above 1 G\(e\) V/\(c^{2}\)  is due to including contributions from open-charm decay, QGP radiation (labeled “QGP+DD”) and multi-pion decays in a consistent way.
The latter is an interesting contribution since the yield might be affected by the degree of \(\rho - a_1\)  mixing.
Note also that the bump at the \(\rho \)  pole mass is explained as being due to primordial \(\rho \)  mesons produced in first chance collisions and escaping the fireball as well as due to decays of \(\rho \)  mesons after chemical freeze-out, the so-called cocktail \(\rho \) .
Overall, thermal radiation makes up only about 50% of the total IMR excess yield.
As discuss already before, the peak and low-mass side of the spectral distribution is well described by the \(\rho \)  contributions with evidence for \(\rho \)  baryon coupling.
The lower-left panel shows again the NA60 data, i.e. for semi-central collisions but here compared to results obtained with HSD and PHSD.
The experimental line shape around the \(\rho \)  pole mass and below is here obtained by explicitly broadening the free \(\rho \)  spectral function (collisional broadening).
The degree of broadening is adjusted as to best reproduce the data.
Also the HSD calculation for hadronic contributions falls short of saturating the yield in the high mass regime.
Only after inclusion of radiation from a plasma phase, included in the calculations, the yield at 1 G\(e\) V/\(c^{2}\)  and beyond is explained.
Note, however, that details about the composition of this part of the radiation is not given.
More details of the calculation can be found in [2]}.
The comparison of the NA60 data to both model calculations clearly disfavor a dropping-mass scenario as already discussed in the previous section (cf. Sec. REF ).
Since the emitting source (fireball) has non-vanishing baryon densities at all energies, and since the \(\rho \)  mesons couple to baryons and antibaryons in the same way, the \(\rho \)  spectral distribution acquires also at SPS a significant broadening (cf. Sec. REF ), yet with less emphasis of the low-mass tail of the spectral function compared to the case at SIS18 energies (HADES).
In the latter case, \(\rho \)  like virtual states in the vicinity of baryons (cloud) decay to virtual photons.
This is the picture behind VDM in baryonic transitions, for which a thorough proof has still to be provided.
Hence, the very low-mass region is of particular interest for studies of baryon-driven effects.
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